1. Field of the Invention
The present invention relates to an imaging device having a sensor in which pixels are arrayed in a matrix, and a driving method therefor and, more particularly, to an imaging device which senses an image associated with radiation such as X-rays entering the sensor, and a driving method therefor.
2. Description of the Related Art
Recently, digital X-ray imaging devices with a photoelectric conversion element are being used to acquire the intensity distribution of X-rays having passed through an object in radiography using, for example, X-rays which are a kind of radiation.
The digital X-ray imaging device is superior in sensitivity and image quality to a conventional film type imaging device. Since the digital X-ray imaging device can save an image as digital data, an image obtained by radiographing (imaging) an object can be processed by various image processes into one which makes diagnosis easier. The digital X-ray imaging device has advantages of making image management easy and allowing transfer of image data using a network.
The X-ray imaging device adopts an X-ray sensor (2D sensor) in which pixels including photoelectric conversion elements and switching elements typified by TFTs are arranged in a 2D array (2D matrix). An object is placed between an X-ray generator and the 2D sensor, and the amount of X-rays having passed through the object is converted into an electrical signal by the 2D sensor, obtaining the X-ray image of the object. Electrical signals (charges) from respective photoelectric conversion elements are independently read out and A/D-converted into digital data by an A/D converter. After the data undergo an image process, the processed data are stored or displayed.
These days, 2D sensors capable of radiographing not only still images but also moving images have been developed. Such 2D sensors are disclosed in, for example, the following patent references 1 and 2. Recently, a demand has arisen for X-ray imaging devices which radiograph not only still images but also moving images by one device in terms of work efficiency and space saving.    Patent Reference 1: Japanese Patent Laid-Open No. 10-285466    Patent Reference 2: Japanese Patent Laid-Open No. 2006-43293
The X-ray imaging device requests high resolutions in still image radiography, and high frame rates of moving images rather than high resolutions in moving image radiography such as fluorography. Even in moving image radiography, the requested resolution and frame rate change depending on the radiographic purpose. For example, neither such a high resolution nor such a high frame rate is required in moving image radiography such as preview for still image radiography. In contrast, high frame rates are required in moving image radiography of a region such as the heart, which moves quickly.
To meet these requirements, in moving image radiography, the frame rate can be increased by reading out pieces of pixel information (charges) at once from a plurality of pixels on the sensor, and adding or averaging the pieces of readout pixel information to increase the image acquisition speed. If pieces of pixel information are independently read out pixel by pixel without executing the addition or averaging process, the frame rate decreases, but a high-resolution image can be acquired.
If pieces of pixel information (charges) of 2×2 pixels are reduced into pixel information of one pixel by the addition process (or averaging process), an image can be acquired at high speed by simply adding (or averaging) and reading out analog signals.
However, if even one of 2×2 pixels is defective, the pixel after the addition process (or averaging process) must be treated as a defective pixel. In this case, the defective pixel greatly degrades a reduced image. The 2D sensor is formed from a semiconductor, and it is generally difficult to prevent generation of any defect in many pixels during the manufacturing process.
It is conceivable to prevent the degradation of a reduced image by performing the reduction process excluding pixel information of a defective pixel. In this case, for example, if pixel information can be acquired as a digital value, it is easy to select only pieces of pixel information of nondefective pixels based on position information of a defective pixel, and add (or average) them. However, in this case, pieces of pixel information of all pixels are A/D-converted, inhibiting an increase in image readout speed (i.e., an increase in frame rate). A special determination mechanism is needed to execute the addition or averaging process excluding a defective pixel before A/D conversion. This complicates the control and circuit configuration.
In short, it has conventionally been difficult to both increase the frame rate and prevent degradation of an image by a simple arrangement.